fairy ring

any of numerous mushrooms of meadows and open woods, especially the well-known Marasmius oreades, that spread in rings originating from mycelial growth: formerly supposed to mark the paths laid by dancing fairies.

fairy ring

noun

1.

a ring of dark luxuriant vegetation in grassy ground corresponding to the edge of an underground fungal mycelium: popularly associated with the dancing of fairies: seasonally marked by a ring of mushrooms